Prana is used to describe the energy in the body as a whole, but the ancient yoga philosophers have also attributed specific names and functions to this energy in certain areas of the body.
The energy in the chest is also known as prana, and is associated with the intake of food, water or air. Prana refers to this nourishment in itself but also to the actions that bring it into the body, the functions of breathing, eating and drinking.
The energy in the lower abdomen is referred to as apana. This energy is associated with elimination from the body. It is both the action of elimination and that which is eliminated. Apana powers elimination of waste upwards, in the exhalation, as well as downwards.
Apana is complementary to prana. Nutrients enter the body under the influence of prana and leave due to effect of apana. If our energies are not in balance, waste (signified by apana) is said to collect in the lower abdomen. This makes us sluggish and heavy. We need to reduce apana to make space for more prana in the body.
In the middle, where prana and apana meet, we have the region of samana energy. Samana is associated with the absorption of nutrients. In this middle area we also find agni, the fire of life. It is said that the inhalation turns the fire downwards to burn the waste and the exhalation turns it upwards to release the waste from the body. This in itself offers an interesting symbolism to explore as we sit and observe the breath.
We can also use this time to bring balance to the inhalation and exhalation. Regardless of our opinions on the existence of prana and agni, it has been shown that a longer exhalation helps to reduce the heart rate and facilitate a sense of relaxation. Start by observing the in-breath and out-breath as they are naturally. You might then go on to gently increase the relative length of the exhalation, to perhaps twice the length of the inhalation. Stay with what feels comfortable and right for you, building the length of the practice over time.